1. Field of the Invention
This invention generally relates to internal combustion engines, and more particularly to a compression release and vacuum release mechanism for four-stoke cycle engines.
2. Description of the Related Art
Compression release mechanisms for four-stroke cycle engines are well known in the art. Generally, means are provided to hold one of the valves in the combustion chamber of the cylinder head slightly open during the compression stroke while cranking the engine during starting. This action partially relieves the force of compression in the cylinder during starting, so that starting torque requirements of the engine are greatly reduced. When the engine starts and reaches running speeds, the compression release mechanism is rendered inoperable so that the engine may achieve full performance. It is normally advantageous for the compression release mechanism to be associated with the exhaust valve so that the normal flow of the fuel/air mixture into the chamber through the intake valve, and the elimination of spent gases through the exhaust valve is not interrupted, and the normal direction of flow through the chamber is not reversed. Examples of compression release mechanisms for four-stroke engines are shown in U.S. Pat. Nos. 3,381,676; 3,496,922; 3,897,768; 4,453,507; 4,977,868; 5,150,674 and 5,184,586, the disclosures of which are incorporated herein by reference. Although known compression release mechanisms are generally effective for relieving compression in the cylinder during cranking the engine, these mechanisms are typically designed to provide compression relief and do not remedy the significant torque established by vacuum in the combustion chamber during the power stroke.
Presently, conventional four-stoke engines require a significant amount of torque to turn the engine over during the power stroke when combustion is not taking place, because the piston is moving downwardly against a pressure difference due to increasing suction or vacuum in the combustion chamber resulting from the partial discharge of gas from the combustion chamber during the immediately preceding compression stroke. The increase of torque required corresponds to a substantial operator or starter force required to drive the piston downwardly against such pressure difference.
In response to the suction torque, one known combustion engine suggests using a contoured cam lobe which acts to hold the valve open longer between the compression and power strokes. Starting torque was decreased by this mechanism, however compression and accordingly engine power would significantly decrease compared to conventional engines which employ the traditional xe2x80x9cpear-shapedxe2x80x9d cam lobes. Yet another known mechanism employs a light spring placed on the stem side of the exhaust valve to hold the valve open during start up. However, in such an arrangement, significant intake and exhaust manifold pressures are required to close the exhaust valve and thus longer times and increased user effort is required to start the engine.
It may be seen that torque, due to compression during start-up, is related to the torque due to vacuum during start-up. Specifically, the release of a significant amount of trapped air during the compression stroke, through the mechanical compression release, causes higher vacuum pressure to form in the cylinder. Very little user effort is required to turn the engine over during the compression stroke, however a substantial starting effort is required during the power stroke. Conversely, though, if very little air is released by the mechanical compression release then beneficially the pressure due to vacuum will be less. However, the pressure caused by compression will be high.
Accordingly, it is desired to provide a release mechanism that addresses the significant torque developed by both the compression and power strokes, is effective in operation, and is relatively simple in construction.
The present invention overcomes the disadvantages of prior internal combustion engines by providing a mechanical compression and vacuum release mechanism which is of simple construction and which significantly reduces the effort required to start the engine. The present compression and vacuum release mechanism includes a centrifugally responsive compression and vacuum release member pivotally mounted to the camshaft, the compression and vacuum release member including compression and vacuum release cams which are in lifting engagement with one of the intake or exhaust valve assemblies of the engine during engine starting to relieve compression and vacuum forces within the combustion chamber and thereby facilitate easier engine starting. After the engine is started and reaches a running speed, the compression and vacuum release member pivots about the camshaft such that the compression and vacuum release cams are disengaged from the lifting engagement with the intake or exhaust valve assemblies for normal engine operation.
In one form thereof, the present invention provides an internal combustion engine, including a cylinder block including a cylinder therein and having a piston reciprocally disposed within the cylinder, the piston operably engaged with a crankshaft; a camshaft in timed driven relationship with the crankshaft; at least one intake valve reciprocally driven by the camshaft; at least one exhaust valve assembly reciprocally driven by the camshaft; and a vacuum release mechanism, including a vacuum release member attached to the camshaft and centrifugally moveable between first and second positions, the vacuum release member including a vacuum release cam extending therefrom, the vacuum release cam in lifting engagement with one of the valve assemblies in the first position during a portion of a power stroke of the piston to relieve vacuum forces opposing the power stroke, the vacuum release cam disposed out of engagement with the one of the valve assemblies in the second position.
In another form thereof, the present invention provides an internal combustion engine, including a cylinder block including a cylinder therein and having a piston reciprocally disposed within the cylinder, the piston operably engaged with a crankshaft; a camshaft in timed driven relationship with the crankshaft; at least one intake valve assembly reciprocally driven by the camshaft; at least one exhaust valve assembly reciprocally driven by the camshaft; and a compression and vacuum release mechanism, including a compression and vacuum release member attached to the camshaft and centrifugally moveable between first and second positions, the compression and vacuum release member including a compression release cam and a vacuum release cam extending therefrom, the compression and vacuum release cams respectively in lifting engagement with one of the valve assemblies in said first position during a portion of a compression and a portion of a power stroke of the piston to relieve compression and vacuum forces respectively opposing the compression and the power strokes, the compression and vacuum release cams disposed out of engagement with the one of said valve assemblies in the second position.
In a further form thereof, an internal combustion engine, including a cylinder block including a cylinder therein and having a piston reciprocally disposed within the cylinder, the piston operably engaged with a crankshaft; a camshaft in timed driven relationship with the crankshaft; at least one intake valve assembly reciprocally driven by the camshaft; at least one exhaust valve assembly reciprocally driven by the camshaft; and a compression and vacuum release mechanism, including a centrifugally actuated common yoke member moveably attached to the camshaft between a first position corresponding to a cranking speed of the engine and a second position corresponding to a running speed of the engine; a compression release cam extending from the yoke member and in lifting engagement with one of the valve assemblies in the second position during a portion of a compression stroke of the piston to relieve compressive forces opposing the compression stroke; and a vacuum release cam extending from the yoke member and in lifting engagement with the one of the valve assemblies in the first position during a portion of a power stroke of the piston to relieve vacuum forces opposing the power stroke; the compression and vacuum release cams disposed out of lifting engagement with the one of the valve assemblies in the second position.